1. Field of the Invention
The present invention relates to a method and apparatus that provide for the controlled delivery of a liquid to a patient. In particular, the invention relates to the controlled delivery of a liquid, preferably including a medication, into a patient with an infusion pump that is operated by gas pressure.
2. Description of the Prior Art
The controlled delivery of liquids, particularly those containing medications, to patients has received substantial attention in the medical arts. The concept of drug infusion is that a patient can be given a medication by intravenous delivery over a given, relatively prolonged, time period. In this manner, the need for repeated injections is eliminated and there is a reduced risk of a development of drug sensitivities. Moreover, it is widely believed that sustained treatment with a drug is generally more effective than single bolus treatment. Further, infusion pump type devices allow for ambulatory treatment of a patient; i.e., the patient need not be attached to an intravenous ("IV") stand and bag.
There have been a number of products in the past which have been useful for delivering liquids, such as medications, at a controlled flow rate. A typical example, which has been quite successful commercially, is illustrated in U.S. Pat. No. 5,080,652 to Sancoff et al. There has been a tendency for developments in this technology to focus on ambulatory care concerns. For example, many devices have been developed primarily for use by a patient. The patient can administer the drug to himself or herself over a prolonged time period without a hospital stay.
Less emphasis has been directed to institutional use (such as use in hospitals). Accordingly, for the most part, the prior art devices have failed to provide for an important need of institutions where long-term storage and subsequent ready availability of medications is important.
Devices such as the previously mentioned Sancoff et al. product have been designed and intended for use shortly after preparation. The devices are filled and soon thereafter connected to the patient, usually through an intravenous tube, and the medication is then administered to the patient by the fluid flow and metering components of the particular device. For instance, in the above-mentioned Sancoff et al. device, the liquid is dispensed or delivered from the device through the action of elasticized membranes which push the liquid containing the medication from the device to the patient. Other products from the prior art use compressed gas to force the medication or other liquid from a container. See, for example, U.S. Pat. No. 5,106,374 to Apperson et al.
Such products, while useful for their purpose of prompt administration of medication to patients, are not amenable to preparation and extended storage of medication for subsequent use. For example, in devices where the medication or other liquid is under constant pressure for an extended period of time, as from a compressed gas or a stressed resilient membrane, the pressure tends to drop as the elastic material loses resiliency or as the compressed gas reacts with the liquid or leaks from the container. Further, such devices generally require complicated valving to retain the liquid under pressure and prevent leakage, which adds significantly to the cost and complexity of the individual products.
Other devices have attempted to circumvent these problems by requiring pressurization at the time that the device is intended to be used. Such devices, however, have been cumbersome and not readily usable. They normally require an external source of pressurization such as attachment to a carbon dioxide cartridge or other outside gas generation equipment. It is time-consuming to obtain such equipment, to connect it to the device, and to wait for the pressurization to be completed. Where medication is needed quickly, the time delay can present a significant danger to a patient.
The prior art has also attempted to make use of on-the-spot gas generation through the use of the reaction of chemicals that generate gas upon contact. See, for example, U.S. Pat. No. 3,023,750, to Baron. The generated gas, then, is used to force a liquid from a bag for delivery to a patient. However, this invention fails to provide the control that is essential to infusion. Gas is generated very rapidly, causing rapid flow rates and high pressure.
A variety of patents for spray type canisters disclose use of chemical reactions to generate a gas for a propellant to drive a liquid component from the canister as an aerosol. In order to avoid the depletion of the reactants in devices such as these, individual tabs of reactants were placed in a plurality of sealed pouches. Over time, the pouches would sequentially dissolve and cause a new reaction to generate additional gas for producing the aerosol. However, this technology would be severely inadequate for use in infusion. Large fluctuations of the pressure inside the canister has been found to render these inventions unsuitable for infusion.
It would therefore be advantageous to have a liquid delivery unit, particularly one for dispensing medications, that can be prepared for use and thereafter have a long storage life without pressurization. In this way, there would be little or no tendency for leakage of the medication or other liquid or loss of pressure potential. It would be additionally advantageous to provide a means for quickly and easily creating a gas propellant which would cause the liquid to be delivered in a controlled manner when and as needed.